The invention relates to the field of microfabricated devices, and in particular to microfabricated devices released to move by removal of a sacrificial layer.
Microelectromechanical systems (MEMS) have a broad range of applications such as, accelerometers, gyroscopes, visual displays and micro-optical systems for fiber-optic communications. The techniques used to form the micromechanical structures, such as surface micromachining, borrow technologies like thin film deposition and photolithography from the microelectronics fabrication industry.
In surface micromachining, thin films of material are typically deposited on a surface (typically known as the handle layer) using a variety of methods to form a device layer of material on a sacrificial layer of material. The micromechanical structure is then formed by patterning and etching the device layer. After the micromechanical structure is formed, a release etch is performed to remove the sacrificial material so that the micromechanical structure is released, allowing it to move and perform mechanical functions.
One actuation scheme used to move the micromechanical structure or otherwise cause it to perform its mechanical function is electrostatic actuation. Electrostatic actuation is commonly used because it does not require complicated fabrication techniques or abnormal materials, such as piezoelectric materials. Electrostatic actuation moves the micromechanical structure by electrostatic attraction between two structures with different voltages applied thereto. When the voltages are applied, the structures move to increase their capacitance by increasing the overlap area of overlapping features, or by closing the gap between the overlapping features.
Because surface micromachining lends itself naturally to creating overlapping surfaces coupled, at least in part, with the common use of electrostatic actuation has resulted in the development of a micromechanical structure used in a number of diverse applications, such as micromirrors, accelerometers, gyroscopes, etc. This structure comprises a plate formed in the device layer that is coupled via flexure assemblies to a frame formed in the device layer. The plate is released to suspend above the handle layer by the removal of the sacrificial layer underlying the plate.
The distance between the plate and the handle layer, however, limits the actuation range of the plate in this structure. This distance directly corresponds to the thickness of the sacrificial layer. An oxide, such as silicon dioxide is typically used as the sacrificial layer. An oxide, however, cannot be grown sufficiently thick to provide the desired actuation range for some applications of this structure.
One such application is micro-optical structures, such as micromirrors. While small deflections suffice for some micromirrors, large micromirrors (greater than about 300 um in diameter) require mirror rotations in the tens of microns (e.g., between about 50–80 um) to be useful. An oxide generally cannot provide for the needed separation between the device layer and the handle layer for such mirror rotations. Therefore, most large micromirrors are not made using the above-described structure. Alternative structures for large micromirrors, such as assembled, hinged or bimorph pop-up structures, have a number of disadvantages. They are often difficult to fabricate, are unreliable, provide low-yield and are many times unmanufacturable devices.
Prior art processes for forming micromirrors also suffer from other disadvantages. For example, many require a through-wafer etch to access the backside of structure. These through-wafer etches create fragile final chips. Etch holes through the mirror surface are often required for the release etch. These etch holes increase signal loss due to scattering. In addition, the prior art processes are not easily integrated with foundry electronics and cannot provide a single chip solution, i.e. one where no assembly is required of separate mirror and electronics chips. The prior art forms micro-optic MEMS systems by constructing the mirror structure on one chip, the electronics on a second chip and then using wire bonding to interface the two components to form the micro-optic system. Integration of active electronics on the same wafer as a micro-optical structure would provide a number of advantages.